The subject matter disclosed herein relates generally to ultrasound probes, and more particularly, to acoustical stacks within the ultrasound probes.
An ultrasound probe typically has many acoustical stacks that each correspond to an imaging element of the probe. Each acoustical stack has several layers that are attached together in a stacked configuration. A piezoelectric layer within the stack is formed of a piezoelectric material, such as piezoelectric ceramic, that has high impedance.
Matching layers are provided on the top side of the piezoelectric layer to transform the acoustic impedances between the piezoelectric layer that has high impedance and an exterior or lens of the probe that has low impedance. The low impedance may be based on the acoustic impedance of water, a human, or other subject to be scanned. Many probes include two matching layers based on quarter-wavelength matching wherein each of the matching layers is approximately one-quarter wavelength thick. Each quarter-wavelength matching layer acts to transform the impedance within a limited bandwidth. Using two quarter-wavelength matching layers limits the bandwidth range to between eighty and ninety percent. To achieve impedance matching in a larger bandwidth, a larger number of quarter-wavelength matching layers is needed. However, increasing the number of quarter-wavelength matching layers greatly increases the thickness of the stack and increases the signal attenuation. In addition, the stacked materials become increasingly difficult to dice, and it may be difficult to find appropriate materials for each of the quarter-wavelength matching layers while still controlling the desired geometry and impedances.
Additionally, a gradient or graded matching layer that uses a material with continuously changing impedance, or many layers of many different materials that have different acoustic impedances, has been proposed for use instead of the discrete quarter-wavelength matching layers. However, these graded matching layer configurations require that the overall thickness of the matching layer(s) be too thick. Good matching characteristics are only achieved if the thickness of the graded matching layer is in the range of at least one or two wavelengths. At this thickness, strong attenuation of the ultrasound signal occurs. The dicing operation is very difficult due to the thickness of the layer, as dicing is difficult for thicker layers but not for thin layers, and requires a high amount of blade exposure. If the graded matching layer has a thickness that is less than one wavelength, however, bad matching or ringing in the bandwidth will result.